The invention relates in general to ion sensors and more specifically to an apparatus, system and method for monitoring an ion concentration of a measured fluid.
Wideband ion sensors are used to measure the concentration of particular ions within a fluid where the fluid may be a gas or liquid. A popular use of wideband gas ion sensors includes using oxygen sensors to determine on oxygen concentration within a gas mixture. Other examples of gas ion sensors include nitrogen sensors that sense gaseous oxides of nitrogen. Many conventional combustion engines utilize oxygen sensors for determining the air to fuel mixture of the exhaust of the combustion engine. Conventional internal combustion engines typically incorporate electronic fueling control using computing devices, such as Electronic Control Units (ECU), that meter fuel into the engine intake depending on engine intake airflow. Typically, the volume of fuel is regulated such that emissions are minimized and all of the fuel is completely burned. The theoretical ratio of air to fuel for complete combustion is 14.7 by weight for gasoline, called the stoichiometric ratio. Theoretically, all available fuel combines with all the intake air at the stoichiometric ratio. The unit Lambda (λ) is often used to represent the quotient of actual air to fuel ratio over the region near the stoichiometric ratio. Conventional electronic fueling systems typically include an oxygen sensor in the exhaust that measures the oxygen concentration of the exhaust. These oxygen sensors act as fuel cells that create an output voltage by combining unburned hydrocarbons in the exhaust with atmospheric oxygen. This results in a lambda/output transfer curve where a λ of 1.0 corresponds to an output voltage of 0.45V. Using the oxygen sensor, the fueling control system regulates the fueling such that the resulting lambda is 1.0 at medium load conditions using a feedback loop. The transfer curve of a typical oxygen sensor is very steep where λ is equal to 1.0, however, and significant variations in output voltage occurs for slight variations in λ. Accordingly, the measured voltage cannot be used to measure other λ values. At high load conditions, a typical internal combustion engine produces maximum power at lambda values<one (0.75 to 0.85). Conventional ECU systems operate in an ‘open loop’ mode under these conditions where the volume of injected fuel is derived solely from pre-stored maps that relate intake air mass to fuel mass without feedback. Because engine aging and production variations change the actual air fuel ratio of the engine, these pre-stored conditions are not always correct for the particular engine. As a result, conventional systems are limited in that severe inefficiencies can occur at high load conditions. Many other wideband ions sensors experience similar drawbacks.
Some recent developments in engine technology have resulted in ‘lean-burn’ systems that operate at lambda ratios greater than 1 (up to 1.1) to minimize fuel consumption and further minimize emissions using special catalysators. Because ordinary lambda sensors are not usable in these lambda regimens, a ‘wide-band’ or Universal Exhaust Gas Oxygen (UEGO) sensor has been developed. UEGO sensors combine a small measurement chamber having an orifice open to the exhaust stream, a standard oxygen sensor (Nernst cell), and a pump cell. The pump cell is a solid-state device of porous ceramic that allows oxygen to move between the atmosphere and the measurement chamber. The direction and magnitude of the current through the pump cell (often referred to as the pump current) determines the direction and flow rate of oxygen ions. In conventional systems, an active feedback loop is incorporated such that the voltage at the oxygen sensor portion of the device is held at the stoichiometric voltage. The pump current can then be used to determine the λ value over a wide range of ratios up to the ratio for free air.
FIG. 1 is graphical illustration of a typical relationship between the pump current and Lambda (λ). As shown in FIG. 1, the resulting curve of pump current vs. lambda value (λ) is non-linear. Although the curve shape does not vary, manufacturing tolerances in the sensors result in different magnitudes of pump current vs. lambda (λ) (i.e. the curve shifts). Attempts to compensate for the variations include incorporating a calibration resistor in the connector to the measuring cell sensor. Unfortunately, this attempted solution does not address all of the variations. Barometric air pressure and exhaust pressure also influence the lambda/pump current relationship. Accordingly, the outputs of theses sensors are not accurate. It is therefore desirable to have a measurement method for oxygen sensors that is self-calibrating and self-compensating for all the above variations.
The pump current vs. lambda curve is also highly temperature dependent. Typical UEGOs contain a heater element that maintains the sensor at the desired operating temperature. The temperature coefficient of the heater element is the quotient of change in resistance (ΔR) to the change in temperature (ΔT). Conventional techniques use the positive temperature coefficient of the heater element to regulate input by operating the element at a constant voltage. Because the temperature coefficient, ΔR/ΔT, is fairly small at the operating temperature, the resulting temperature regulation is not very precise. Depending on the sensor, the pump cell impedance, the Nernst cell impedance, or both have a much bigger temperature coefficient, ΔR/ΔT, and would, therefore, allow more precise temperature control. It would be more advantageous to control the temperature of the pump cell. Unfortunately, at lambda values near 1, the pump current is very small or equal to zero and the pump cell impedance can not be accurately measured on a low current. The Nernst cell is typically physically bonded to the pump cell and, therefore, the temperature of the Nernst cell and the pump cell differ by a small amount. In order to measure the Nernst cell impedance, a known fixed current or known fixed voltage have to be impressed on the Nernst cell and the resulting voltage or current then measured. Alternatively, a small alternating current (AC) voltage or current can be impressed on the Nernst cell and the resulting AC impedance measured. The first method requires stopping the lambda measurement for a period of time and also requires impressing the reverse charge on the Nernst cell to speed up recovery. The second method does not interfere with the measurement but requires low pass filters to remove the AC voltage or current from the measured signal. The filters also remove the higher signal frequencies which results in an inability to detect short transient responses. Both methods measure the temperature of the Nernst cell, not the pump cell. During operation, a temperature gradient between the pump cell and the Nernst cell may occur and some temperature control errors may result. Therefore there is a need for precise pump cell temperature control while measuring lambda without resorting to complicated circuitry to remove measurement artifacts.
Further, conventional fuel metering techniques result in significant pollution during the warm up period of the oxygen sensor. In conventional systems where UEGO sensors are used, a precise operating temperature must be attained before the UEGO output value is reliable. This increases the time the fuel injection systems runs in ‘open loop’ without knowledge of actual air-fuel ratio. As a result, the time the engine creates uncontrolled warm-up pollution is dependent on the sensor warm-up time. Therefore, there also exists a need for an apparatus, system and method for measuring an oxygen concentration which minimizes the time before a reliable value is produced by the sensor.
Current wideband ion sensors such as wideband oxygen sensors (WBO2 sensors) combine a Nernst cell reference sensor and a pump cell in single package. A Nernst cell is an electrochemical cell that produces a voltage that is nonlinearly proportional to the difference in partial pressure of a measured gas between electrodes of the cell. In a typical oxygen sensor application, the electrodes are exposed to atmospheric air on an electrode on one side of a measuring chamber and to an exhaust gas of an internal combustion engine on the other electrode. A voltage is created by oxygen ions migrating through the solid electrolyte material of the cell. The pump cell is a Nernst cell where oxygen ion flow through the cell is forced by an electrical current. If the current flows in one direction, oxygen ions are transported from the outside air into the sensor. If the current is reversed to the other direction, oxygen ions are transported out of the sensor to the outside air. The magnitude of the current determines the number of oxygen ions that are transported each second.
The Nernst voltage is a voltage created as result of electrochemical reaction in the cell. The cell acts basically as a fuel cell. The Nernst voltage is created by the difference in oxygen partial pressure between the two electrodes of the cell. The Nernst equation describes it:Voutput=(R*)(T)/(n)(F)*ln [(Po,air)/(Po,exh)]
where,
Voutput=O2 sensor's output voltage (0 to 1.0 volt is a typical range)
R*=Universal Gas Constant=8.3143 [Joule/gram-mole*K]
T=Temperature of the exhaust gas [Deg K]
n=number of electrons involved in the reaction=4 in the NBO2 case
F=Faraday constant=96,480 [Coulomb/gram-mole]
Po, air=Partial pressure of O2 in the atmosphere [Pascals]
Po, exh=Partial pressure of O2 in the exhaust gas at temp [Pascals].
In conventional systems, both the Nernst cell and the pump cell are mounted in a very small measurement chamber open with an orifice (diffusion gap) to the exhaust gas. During a rich condition, there is little or no oxygen and relatively high levels of oxidizable combustion products within the measuring chamber. In rich conditions, the WBO2 controller regulates the pump cell current such that just enough oxygen ions are pumped into the chamber to consume all oxidizable combustion products. This action basically produces a stoichiometric condition in the measurement chamber. In the stoichiometric condition, the Nernst reference cell produces 0.45V. In a lean condition where there is excess oxygen, the controller reverses the pump current so that all oxygen ions are pumped out of the measurement chamber and a stoichiometric condition returns. The pump cell is strong enough to pump all oxygen out of the measurement chamber even if the chamber is filled with free air.
The task of the WB controller in conventional systems, therefore, is to regulate the pump current such that there is never any oxygen nor oxidizable combustion products in the measurement chamber. The required pump current is a measure of the Air/Fuel ratio. Conventional wideband sensors, however, are difficult to produce because multiple cells are combined in a small package. Also, the small orifice to exhaust gas is susceptible to contamination or blockage by exhaust particles limiting performance of the sensor. In addition, conventional wideband sensors exhibit a delay between Nernst reference cell output and changing pump cell current because of the physical separation between the two devices. Accordingly, an improved ion sensor is needed.